The present invention relates generally to the field of analog and mixed signal processing integrated circuits and operational amplifiers. More particularly, the present invention relates to dynamic cascoding technique for operational amplifiers.
Operational amplifiers (op amps) often include a widely used technique called cascoding in order to increase output impedance and to reduce the effect of Miller capacitance. By cascading transistors in op amps powered with high voltages, such as those used in flash memory chips, the impact ionization current through the cascoded transistors can also be reduced. However, these advantages come at the expense of the common mode range of the op amp. Normal cascading can reduce the common mode range.
The gates of the cascode transistors are driven from a fixed DC voltage in traditional cascoding techniques. This arrangement causes the cascode transistors to turn off as the common mode input voltage of the differential amplifier is about a threshold voltage from the gate voltage of the cascode transistors. As a result, the differential pair is also turned off. Thus, the upper end or the lower end of the common mode voltage range is reduced, depending on whether the input transistors of the differential pair are N type or P type transistors.